The invention relates to a method for ultrasonic bonding, wherein two longitudinal ultrasonic waves propagating in a common working plane oriented parallel to a substrate are induced in one or more ultrasonic transducers and are passed through ultrasound-conducting arms from different spatial directions onto a common connection point of the arms, and at the connection point cause an ultrasonic tool arranged directly or indirectly at said connection point to oscillate in two or three dimensions.
In addition, the invention relates to a device for carrying out the bonding method.
Known from EP 1,317,990 A1 is a so-called ultrasonic cross-transducer having at least two ultrasonic transducers associated with a common working plane for generating two ultrasonic longitudinal waves in a working plane oriented parallel to a substrate. The ultrasonic waves generated in the ultrasonic transducers are guided by arms that are assigned to the ultrasonic transducers and that are conducting the ultrasound from different spatial directions onto a connection point of the arms and from there are passed on directly or indirectly via a connection element to an ultrasonic tool. Preferably, the ultrasonic tool protrudes at a right angle from the working plane of the ultrasonic transducer in the direction of the substrate. The tool is induced into bending oscillations. Due to the bending oscillation, a tool tip facing the substrate carries out a movement, which is transferred contact-based to a component, for example a semiconductor chip or a bond wire, that is to be connected to the substrate. The friction movement between substrate and component creates a metallic connection by friction welding in the contact area. By the angular offset arrangement of the ultrasonic transducers to each another—preferably four ultrasonic transducers are provided that are arranged at respective 90° angles to each other—a two-dimensional ultrasonic oscillation can be generated in the working plane and transferred to the component. While in one-dimensional ultrasonic bonding methods a time-varying relative speed and acceleration occurs between substrate and component, and a transition from static friction to sliding friction occurs at every point of a movement-reversal, when using the ultrasonic cross transducers, a constant speed or acceleration amount between the substrate and component can be reached and a constant change between static and sliding friction can be avoided. This reduces the mechanical stress on the components that are to be bonded with the substrate, making the method particularly well suited for bonding of dedicate semiconductor chips. Because of the no longer existing static to sliding transitions, a higher mechanical output power is available for carrying out the welding process using the same electrical power input. The same friction energy required for the bonding formation can then be achieved with smaller amplitudes in less time. This is particularly advantageous because the total friction work to be performed increases as well in the flip chip technology as a result of an increase of the electrical contacts per chip, and therefore, the welding power must be increased at a constant cycle time in order to weld chips having a large number of electrical contacts. The work carried out during a cycle of the harmonic oscillation of the system at the welding point is proportional to the radius of the circle or to the semi-axes of the ellipses, respectively, of the self-adjusting movement, and these are limited by the maximum achievable amplitude of the ultrasonic transducers.